Synthesis of Triphenyl Phosphate and Benzyl Benzoate with Phase-Transfer Catalyst in Heterogeneous Liquid-Liquid Reaction System S a t o r u Asai,’ Hidemi Nakamura, Mitsunori Tanabe, and Kenji Sakamoto Department of Chemical Engineering, University of Osaka Prefecture, Sakai, Osaka 593,Japan
The synthetic reactions of triphenyl phosphate from diphenylphosphoryl chloride and sodium phenoxide and of benzyl benzoate from benzyl chloride and sodium benzoate with phase-transfer catalyst were studied in a heterogeneous liquid-liquid reaction system using an agitated vessel with a flat interface. Tetrabutylammonium bromide and 1,2-dichloroethane were used as a catalyst and a solvent, respectively. The behavior of the observed overall reaction rates was explained well by the proposed model. The overall reaction rates were proportional to the organic-phase interfacial concentrations of the actual reactants, tetrabutylammonium phenoxide for the synthesis of triphenyl phosphate and tetrabutylammonium benzoate for that of benzyl benzoate. Their interfacial concentrations were a unique function of the concentrations of tetrabutylammonium bromide and the respective sodium salts. The intrinsic reaction rate constants, for the synthetic reactions of triphenyl phosphate and benzyl benzoate, which were evaluated by fitting the rate data to the model prediction, were 2.33 X lo6 and 2.56 m3/kmol-s at 303 K, respectively. Introduction Phase-transfer catalyst can enhance the rates of heterogeneous liquid-liquid reactions which are unreactive at ordinary temperatures and pressure. The previous studies have been carried out with a view to obtaining high product yield and high selectivity (Weberand Gokel, 1977; Starks and Liotta, 1978; Dehmlow and Dehmlow, 1983), and the approach from the aspect of chemical engineering allowing for the effect of mass transfer on the overall reaction rate has rarely been achieved. We carried out the alkaline hydrolysis of n-butyl acetate and the oxidation of benzyl alcohol by using phase-transfer catalysts and demonstrated that the overall reaction rates could be explained well by the theoretical solution for phase-transfer catalysis with mass transfer (Asai et al., 1992, 1994). In the present work, we analyzed the synthetic reactions of two esters, that is, triphenyl phosphate from diphenylphosphoryl chloride and sodium phenoxide (system 1) and benzyl benzoate from benzyl chloride and sodium benzoate (system 2). Triphenyl phosphate (system 1)is indispensable for the plastic industry as a plasticizer or flame retarder of polymer (such as polyvinyl chloride, cellulose acetate, etc.). Until now, triphenyl phosphate has been synthesized by the reaction of phosphoryl chloride with phenol under elevated temperature and pressure by using aluminum chloride or magnesium chloride as a solid catalyst. However, its severe operating conditions and the corrosion of the reactor by hydrogen chloride formed as a byproduct are the disadvantages of this process. On the contrary, the use of phasetransfer catalyst, with sodium phenoxide instead of phenol as a reactant, makes it possible for the reaction to proceed under mild conditions and to avoid the production of hydrogen chloride. It has been confirmed that the synthetic reaction of triphenyl phosphate between phosphoryl chloride and sodium phenoxide occurs in consecutive steps as follows (Krishnakumar and Sharma, 1985):
Reaction I11 is a rate-determining step. Krishnakumar and Sharma carried out the experiments on the reaction I11 by using Aliquat 336 as a phase-transfer catalyst and chloroform as a solvent. They suggested that the overall reaction rates were explicable qualitatively by the theoretical predictions for the fast pseudo-first-order reaction. However, the evaluation procedure of the interfacial concentration of the actual reactant, that is, the ion pairs consisting of quaternary ammonium cation and phenoxide ion,...